Rechargeable lithium-ion (Li-ion) and lithium metal batteries (including Li-sulfur and Li metal-air batteries) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li4.4Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries have a significantly higher energy density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having relatively high specific capacities, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode through the electrolyte to the cathode, and the cathode became lithiated. Unfortunately, upon repeated charges/discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately grew to penetrate through the separator, causing internal shorting and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's, giving ways to lithium-ion batteries.
In lithium-ion batteries, pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1.
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-180 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range of 120-240 Wh/kg, most. These specific energy values are two to three times lower than what would be required for battery-powered electric vehicles to be widely accepted.
With the rapid development of hybrid (HEV), plug-in hybrid electric vehicles (HEV), and all-battery electric vehicles (EV), there is an urgent need for anode and cathode materials that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. Two of the most promising energy storage devices are the lithium-sulfur (Li—S) cell and lithium-selenium (Li—Se) cell since the theoretical capacity of Li is 3,861 mAh/g, that of S is 1,675 mAh/g, and that of Se is 675 mAh/g. Compared with conventional intercalation-based Li-ion batteries, Li—S and Li—Se cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). With a significantly higher electronic conductivity, Se is a more effective cathode active material and, as such, Li—Se potentially can exhibit a higher rate capability.
However, Li—Se cell is plagued with several major technical problems that have hindered its widespread commercialization:    (1) All prior art Li—Se cells have dendrite formation and related internal shorting issues;    (2) The cell tends to exhibit significant capacity decay during discharge-charge cycling. This is mainly due to the high solubility of selenium and lithium poly selenide anions formed as reaction intermediates during both discharge and charge processes in the polar organic solvents used in electrolytes. During cycling, the anions can migrate through the separator to the Li negative electrode whereupon they are reduced to solid precipitates, causing active mass loss. In addition, the solid product that precipitates on the surface of the positive electrode during discharge becomes electrochemically irreversible, which also contributes to active mass loss. This phenomenon is commonly referred to as the Shuttle Effect. This process leads to several problems: high self-discharge rates, loss of cathode capacity, corrosion of current collectors and electrical leads leading to loss of electrical contact to active cell components, fouling of the anode surface giving rise to malfunction of the anode, and clogging of the pores in the cell membrane separator which leads to loss of ion transport and large increases in internal resistance in the cell.    (3) Presumably, nanostructured mesoporous carbon materials could be used to hold the Se or lithium polyselenide in their pores, preventing large out-flux of these species from the porous carbon structure through the electrolyte into the anode. However, the fabrication of the proposed highly ordered mesoporous carbon structure requires a tedious and expensive template-assisted process. It is also challenging to load a large proportion of selenium into these mesoscaled pores using a physical vapor deposition or solution precipitation process. Typically the maximum loading of Se in these porous carbon structures is less than 50%.
Despite the various approaches proposed for the fabrication of high energy density Li—Se cells, there remains a need for cathode materials, production processes, and cell operation methods that retard the out-diffusion of Se or lithium polyselenide from the cathode compartments into other components in these cells, improve the utilization of electroactive cathode materials (Se utilization efficiency), and provide rechargeable Li—Se cells with high capacities over a large number of cycles.
Most significantly, lithium metal (including pure lithium, lithium alloys of high lithium content with other metal elements, or lithium-containing compounds with a high lithium content; e.g. >80% or preferably >90% by weight Li) still provides the highest anode specific capacity as compared to essentially all other anode active materials (except pure silicon, but silicon has pulverization issues). Lithium metal would be an ideal anode material in a lithium-selenium secondary battery if dendrite related issues could be addressed.
Sodium metal (Na) and potassium metal (K) have similar chemical characteristics to Li and the selenium cathode in sodium-selenium cells (Na—Se batteries) or potassium-selenium cells (K—Se) face the same issues observed in Li—S batteries, such as: (i) low active material utilization rate, (ii) poor cycle life, and (iii) low Coulumbic efficiency. Again, these drawbacks arise mainly from insulating nature of Se, dissolution of polyselenide intermediates in liquid electrolytes (and related Shuttle effect), and large volume change during charge/discharge.
Hence, an object of the present invention is to provide a rechargeable Li—Se battery that exhibits an exceptionally high specific energy or high energy density. One particular technical goal of the present invention is to provide a Li metal-selenium or Li ion-selenium cell with a cell specific energy greater than 300 Wh/Kg, preferably greater than 350 Wh/Kg, and more preferably greater than 400 Wh/Kg (all based on the total cell weight).
It may be noted that in most of the open literature reports (scientific papers) and patent documents, scientists or inventors choose to express the cathode specific capacity based on the selenium or lithium polyselenide weight alone (not the total cathode composite weight), but unfortunately a large proportion of non-active materials (those not capable of storing lithium, such as conductive additive and binder) is typically used in their Li—Se cells. For practical use purposes, it is more meaningful to use the cathode composite weight-based capacity value.
A specific object of the present invention is to provide a rechargeable lithium-selenium cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with conventional Li—Se cells: (a) dendrite formation (internal shorting); (b) low electric and ionic conductivities of selenium, requiring large proportion (typically 30-55%) of non-active conductive fillers and having significant proportion of non-accessible or non-reachable selenium or lithium polyselenide); (c) dissolution of lithium polyselenide in electrolyte and migration of dissolved lithium polyselenide from the cathode to the anode (which irreversibly react with lithium at the anode), resulting in active material loss and capacity decay (the shuttle effect); and (d) short cycle life.
In addition to overcoming the aforementioned problems, another object of the present invention is to provide a simple, cost-effective, and easy-to-implement approach to preventing potential Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal-selenide batteries.